find_on_path on systems with tight permissions

I'm using setuptools on a server with tight permissions; a normal user is not allowed to list the contents of certain directories, including /usr/bin, which setuptools tries to scan when searching for .egg files. Thus, running any command installed from setuptools fails with

OSError: [Errno 13] Permission denied: '/usr/bin'

The following change in find_on_path(), in pkg_resources.py, makes it ignore directories which it cannot read: